1. Field of the Invention
The present invention relates to a method of waste treatment and, more particularly, to the treatment of animal waste in the form of a slurry produced, for example, by hogs and dairy cattle.
2. Description of the Related Art
In modern agricultural production, it has become usual to employ an agricultural establishment as an intensive feeding operation for raising hundreds or even thousands of animals, e.g. hogs, dairy, poultry and beef animals, at a single location. Consequently, large amounts of animal waste matter are produced at such locations, and the disposal of this waste matter in a hygienic and inoffensive manner is problematic.
More particularly, it has been suggested that hogs produce more than four times as much waste as humans. It has, for example, been planned to initiate a hog farm containing 2,000,000 hogs, which will produce an amount of waste equivalent to that produced by the city of Los Angeles. The current pig population of North Carolina is four times that number. Recent accidents in North Carolina, in which millions of gallons of pig excreta were released into the countryside, have drawn attention to the problems involved in the disposal of such waste. Applications for permits to construct large piggeries have been denied because of concerns about odours and waste disposal.
Animal waste, including hog manure, has traditionally been disposed of by spreading it over agricultural land as a liquid fertilizer and, for sufficiently large cropped land bases, this method of disposal can be performed safely. However, in areas of high population density or where multiple farms are closely clustered, and the land base is therefore relatively small, problems arise. For example, excess nutrients cannot be absorbed by crops and leach into ground water and surface water.
When land application is prevented, e.g. because the ground is frozen or saturated with water and/or nutrients, the waste must be stored, usually in large pits, with consequential costs and environmental risk.
As an alternative to land disposal, it is also common practice to compost the waste. For this purpose, the waste is separated into solid and liquid by the use of inclined screens, sometimes followed by the use of belt or filter presses. These methods produce an output containing about 45% solid material at best, and this is then mixed with a bulking material to facilitate aeration and an appropriate C:N ratio, and deposited in windrows for stabilization, over a period of about 45 to 60 days, by composting. This process does not ensure that pathogens in the waste are killed and the windrows are often odorous and may attract birds, rodents and vermin. The liquid still has to be dealt with through land application.
The challenges involved in the treatment of animal waste include the destruction of pathogens, the controlled extraction of ammonium and plant nutrients, and the disposal of organic matter.
It is known, from U.S. Pat. No. 4,093,544, issued Jun. 6, 1978, to David S. Ross, the disclosure of which is incorporated herein by reference, to remove ammonium from wastewater by firstly increasing the pH value of the wastewater, then desorbing ammonium by vacuum and subsequently absorbing the desorbed ammonium in liquid having a pH value and temperature substantially less than that of the original wastewater.
In U.S. Pat. No. 5,914,040, issued Jun. 22, 1999, to Yvette Pescher et al., there is disclosed a process for the purification of a medium containing organic waste, for example pig manure, by subjecting the medium to treatment with an oxidizing agent, treatment with a flocculating agent and treatment with at least one polyelectrolyte. The effluent thus obtained is filtered to provide a filtration cake and a filtrate.